Seat belts are known to have two basic functions during a motor vehicle accident. The first function is a restriction of movement for an occupant wearing the seat belt and the second function is dispersion or dissipation of energy in as safe a manner as possible during the accident. Stated differently, seat belts, and other vehicle safety equipment such as airbags, are designed to restrict the movement of the occupant(s) within the motor vehicle during an accident and prevent or lessen the severity of impact between the occupant(s) and other objects within the interior of the vehicle.
Several types of seat belts are used, or have been used, in modern cars and trucks. Three common types of seat belts are lap belts that extend across an occupant's lap, shoulder belts that extend across an occupant's torso, and combination lap and shoulder belts. Seat belts can also be described by the number of “anchor points” used to attach the seat belt to the vehicle. For example, a lap belt can be referred to as a “two-point” seat belt that is anchored at two locations within the vehicle and on opposite sides of the occupant's hips. In addition, the anchor points can include the seat structure, the occupant's door, vehicle floor and the like. Also, a lap and shoulder belt can be known as a “three-point” seat belt that has two anchor points on opposite sides of an occupant's hips and one anchor point on a frame of the vehicle proximate a shoulder of the occupant, e.g. a B pillar.
Presently, U.S. federal law requires that manufacturers provide three-point safety belts that have a lap belt portion and a shoulder harness portion for all seats in new motor vehicles. As stated above, the lap belt portion extends across the lap of the occupant while the shoulder harness portion typically extends diagonally across a torso of the occupant. In addition, a pretensioner is typically provided, the pretensioner pulling the seat belt tight in the event of a collision and thereby taking up any extra slack and reducing any force the occupant's body is subjected to during the collision.
The pretensioner typically has a force or load limit at which it will allow at least part of the seat belt to slip or extend therefrom in order to disperse or dissipate energy during the collision. For example, current pretensioners have a force or load limit generally between 2 and 3 kilonewtons, which is a function of the force a hip and/or lower spine of an average individual can withstand without serious injury. Once the limit exceeded, the pretensioner affords for in slippage or extension of at least a portion of the seat belt therefrom, which results in the occupant moving forward in the event of a front-end collision and possibly impacting another energy absorber device such as a front airbag, collapsible dashboard, etc.
If a seat belt could have a higher pretensioner force or load limit and not result in serious injury to the occupant's hips, lower spine, etc., forward movement of the occupant during a front-end collision could be reduced and thereby result in a less forceful impact between the occupant and the front airbag, collapsible dashboard, etc. Therefore, a seat belt assembly that provides a higher pretensioner force or load limit without serious injury to an occupant wearing the seat belt assembly would be desirable.